Generally, the industry of semiconductor manufacturing involves highly complex techniques for fabricating integrating circuits using semiconductor materials which are layered and patterned onto a substrate, such as silicon. Due to the large scale of circuit integration and the decreasing size of semiconductor devices, the fabricated devices have become increasingly sensitive to defects. That is, defects which cause faults in the device are increasingly smaller. The device needs to be fault free prior to shipment to the end users or customers.
An integrated circuit is typically fabricated from a plurality of reticles (also referred to as “photomasks” or “masks”). Generation of reticles and subsequent inspection of such reticles have become standard steps in the production of semiconductors. Initially, circuit designers provide circuit pattern data, which describes a particular integrated circuit (IC) design, to a reticle production system, or reticle writer. The circuit pattern data is typically in the form of a representational layout of the physical layers of the fabricated IC device or die. The representational layout includes a representational layer for each physical layer of the IC device (e.g., gate oxide, polysilicon, metallization, etc.), wherein each representational layer is composed of a plurality of polygons that define a layer's patterning of the particular IC device.
The reticle writer uses the circuit pattern data to write a plurality of reticles that will later be used to fabricate the particular IC design. For example, an electron beam writer or laser scanner may be used to expose a reticle pattern. A finished reticle or photomask typically has at least transparent and opaque regions, and sometimes semi-transparent and phase shifting regions, which together define the pattern of coplanar features in an electronic device such as an integrated circuit. Reticles are used during photolithography to define specified regions of a semiconductor wafer for etching, ion implantation, or other fabrication processes.
After fabrication of each reticle or group of reticles, each new reticle typically is free of defects or degradation, but sometimes has defects that were introduced during fabrication. A reticle inspection system may then be used to inspect the reticle for defects that may have occurred during the production of the reticles. However, the reticle may become defective after use. Thus, there is a continuing need for improved reticle inspection techniques and apparatus.